One could very well argue "yes", but first and foremost speaking the language of their offline communities is pretty likely, if only through sheer inertia and a lack of concrete reasons to question that choice.
At 18, one stands a very good chance of having spent one's entire life roughly within the confines of a single monoculture and it's morals, social pressures, religion, language, etc etc etc. - quite possibly with contact intentionally limited by one's parents (not all of us were blessed/cursed with unrestricted, unmonitored, and unfiltered internet access, and many parents do a lot of meddling when it comes to friends and other social connections as well.)
It's often shortly after, when said people go to college, that they experience their first true culture shock. To the point where some communities will outright discourage college (pressuring boys to inherit a trade, or girls to marry and become homemakers), or at least discourage going out-of-state, pressuring children to stay closer where they can be - if not controlled - at least kept in a similar social environment, where they obstensibly won't "lose their way", where they can be more readily influenced.
Not saying one can't break away at 18, but for many people, the question isn't "Why [become a mormon missionary]?" but "Why break with tradition, family, friends, and one's entire support network, just to skip a little missionary work?"
If your entire community holds the same assumptions (or at least publicly claims to), and you haven't been exposed to meaningful dissent and opposing viewpoints to challenge those assumptions, one is likely to uphold the same assumptions without ever having meaningfully questioned them, or ever having made any kind of active "decision" about said assumptions.
To take a non-religious example, many of us grew up being taught and believing there were 9 planets in our solar system, and most of us accepted that without ever having meaningfully questioned the taxonomical decisions going into what exactly defines a "planet" versus smaller planetoids and how different Pluto might be from Ceres. We were born into a culture that seeded our assumptions, and we had no great need or reason to overturn them.
This doesn't mean we can't break away from the culture we were born into given sufficient resources and/or reason (one can form dissenting opinions, one can move, ...) but it often explains default behavior, and what one might chose if they don't even realize they're making a choice, if only through inaction.